features/virtualization: update kvm-unit-tests doc

Update the kvm-unit-tests documentation to allow running tests on the
buildroot filesystem rather than running on a Linux distribution. The
kvm-unit-tests now include a standalone test script to run tests in
non-secure state that is used on the buildroot filesystem.

Signed-off-by: Vivek Gautam <vivek.gautam@arm.com>
Change-Id: I44531e99c03b2258533ea27fb2d0026a15795953